Tests the specified descriptor to determine if it is a 64-bit
    format descriptor, and directs the program flow based on the
    outcome of the test.

    Format

      $IS_DESC  desc_addr, target, size=long | quad

1 - Parameters

 desc_addr

    The address of the descriptor to test.

 target

    The label to branch to if the descriptor is in 64-bit format.

 size=long|quad

    The size of the address pointing to the descriptor. The default
    value is size=long.

2 - Description

    $IS_DESC64 tests the fields that distinguish a 64-bit descriptor
    from a 32-bit descriptor. If it is in 64-bit form, a branch is
    taken to the specified target. The address to be tested is read
    as a longword, unless size=quad is specified.

3 - Examples

    1.$is_desc64 r9, 10$

      In this example, the descriptor pointed to by R9 is tested, and
      if it is in 64-bit form, a branch to 10$ is taken.

    2.$is_desc64 8(r0), 20$, size=quad

      In this example, the quadword at 8(R0) is read, and the
      descriptor it points to is tested. If it is in 64-bit form,
      a branch to 20$ is taken.